Internal Combustion engines generally, and compression ignition diesel engines specifically, produce particulate matter which is considered a waste byproduct of combustion and harmful to the environment. Governments around the world have created regulations focused on reducing the level of Particulate Matter (PM) along with Nitrous Oxides (NOx), Unburned Hydrocarbons (UHC) and Carbon Monoxide (CO). Since the formation of NOx is inversely proportional to the particulate matter being created, many technical achievements such as ultra-high fuel injection pressure systems have been developed in order to allow NOx emissions reduction technologies while maintaining or reducing particulate matter emissions. The ultra-high injection pressure (e.g. 30,000 psi or higher) creates better atomization of the fuel so that the limited oxygen available with NOx reducing technologies such as Exhaust Gas Recirculation (EGR) can be utilized for lower engine out NOx while maintaining or reducing particulate matter emissions. While ultra-high injection pressure reduces particulate matter emissions, the parasitic load on the engine is increased along with an increase in engine system cost.
It should be noted that while the particulate mass has dropped, an increase in the number of engine out ultrafine particles increases the health risk of internal combustion engines exhaust. In order to reduce the particulate exiting the tailpipe, the current commercial state of the art engine technology has typically included a Particulate Filter (PF) to trap the particles in the engine's exhaust before being released into the atmosphere. While particulate filters have been commercially available for decades, the technologies for removing the built up particulate matter have had varying degrees of success depending on the operating cycle of the engine. This along with fuel efficiency reductions caused by the filter restriction and regeneration thermal energy, that provides no useable output work, has required government regulations to bring about the main stream commercial availability of the technology.
Commercially available active exhaust treatment systems utilize a particulate filter which is thermally regenerated. Fuel is a convenient source of energy for such regeneration. During active filter regeneration, the exhaust gas temperature can be increased by combusting an additional quantity of fuel in the exhaust system using specialized hardware and using one of the following methods:
Late injection combustion—Fuel is injected later in the expansion stroke so that the lower effective compression ratio produces high exhaust temperatures
Flame combustion—the fuel is combusted in a fuel burner, usually with a dedicated supply of combustion air, with the flame entering the exhaust system.
Catalytic combustion—the fuel is introduced through an exhaust injector, evaporated and mixed with exhaust gas, and oxidized over an oxidation catalyst.
Combined flame and catalytic combustion—a combination of the above methods, where a fuel burner is followed by a catalytic combustion system.
Further details can be found in “Filters Regenerated by Fuel Combustion” by W. Addy Majewski. In short, the removed particulate is burned and creates CO2 which is passed into the atmosphere.
The current thermal solutions, described above, are overly complicated, require some method of active thermal regeneration, or require a high exhaust temperature operating cycle for thermal regeneration. These active thermal regeneration technologies utilize additional fuel use for increasing exhaust temperature which does not provide useable output work. The use of fuel, without subsequent output work, does not comply with the current global concern for Green House Gas, (GHG) Carbon Dioxide (CO2) emissions or the end user concerns over high fuel prices (operating costs). In addition to utilizing fuel, the current systems require sophisticated control algorithms, sensors, burners or dosing systems, and typically utilize finitely available and costly rare earth elements. The sustainability of such technologies is in question. U.S. Pat. No. 7,992,382 describes using a back flow of filtered exhaust gases to regenerate the filter non-thermally. Utilization of the arrangement does not improve the NOx effectiveness of the catalyzed SCR systems.
To reduce NOx emissions, the current approach is to place a Selective Catalytic Reduction, (SCR) in the exhaust system to reduce the NOx to Nitrogen and water. This requires exhaust temperatures ranges that do not cover the entire engine operating range and do not operate effectively until they have reached temperatures above their light off temperature. Since these technologies require elevated operating temperatures typically above 250 degrees Celsius, they do not perform at start up or during low temperature stop and go urban operation. This problem is exacerbated due to current active thermal PF regeneration technologies needing partial passive regeneration to keep the fuel used for active regeneration and subsequent overall brake specific fuel consumption (BSFC) to a minimum. The current approach for passive regeneration is to use NO2 which was converted from NOx by the rare earth metals in the Oxidation Catalyst (DOC). Since NOx is only present, in sufficient quantity, before the SCR, the PF must be placed upstream of the SCR for passive thermal regeneration by NO2 to occur. The upstream position of the PF creates a heat sink in the system and reduces the rate at which the downstream SCR temperature is ramped up during startup and urban drive cycles. These first few minutes significantly contribute to the overall cycle emissions of the vehicle subsequently requiring aftertreatment that is typically greater than 90% efficient. The current high NOx effectiveness requirement, along with the removal of costly precious metals utilized in lean NOx traps, has made SCR the main technology for mobile and stationary engines.
The current active thermal system approach has added additional constraints to the SCR catalyst that can be implemented, due to the high temperature durability concerns for SCR catalyst such as Vanadium Pentoxide. Copper Zeolite catalysts are currently utilized for SCR solutions that include thermal regeneration, while operating with ultra-low sulfur fuels, due to the catalyst's high temperature capability and reasonable sulfur tolerance to this low level of fuel sulfur. Copper zeolite catalyst, even while utilizing ultra-low sulfur (<15 ppm) fuels, require a rich air/fuel ratio sulfur regeneration event to desorb the sulfur utilizing greater than 700° Celsius temperatures. This periodic sulfur desorption regeneration requires a method of attaining a high temperature in the SCR, whether it be an oxy-cat or full burner regeneration in order to retain their performance. The regeneration requirement adds cost and complexity to the system and reduces the engine's overall thermal efficiency. Additionally, the use of exhaust energy recovery systems, are constrained by the maximum temperature required for this regeneration event to occur. The elimination of this thermal desulfation event would be a desired thermal efficiency improvement.
Additionally, the close-coupled nature of current catalyst arrangements limit the residence time for complete urea hydrolysis making the potential for urea deposits on the catalyzed PF substrate and exhaust piping. The depositing of the urea on the particulate filter has been shown to have additional adverse effect on the NOx effectiveness after aging. In order to attain ultra-low emissions at temperatures below 200 degrees Celsius, the use of a static mixer, advanced injectors, or an increase in the distance between the injection site and the catalyst is required. Hydrolysis requires temperature and time in order to be completed. Increased distance between the urea injection site and the SCR catalyst will further improve the effectiveness of the urea hydrolysis without adding complexity to the system by means of high urea injection pressures or the aforementioned static mixer.
Thermal aging of SCR systems is a known problem that is significantly increased by the active thermal regeneration of the particulate filter and sulfur desorption and one that requires an oversized substrate or additional catalyst material in order to achieve acceptable aged catalyst state NOx effectiveness. Additional catalyst, in the coating, increases the backpressure created by the substrate along with additional cost. While catalyst companies have been attempting to achieve higher temperatures with vanadium pentoxide catalyst, their operation still does not have the thermal durability to withstand the high temperature requirements of thermal regeneration compared to copper zeolite. The removal of high temperature regeneration for longer aftertreatment life, low temperature NOx effective catalyst, and exhaust energy recovery is desired.
The ash accumulation, in thermal systems, that utilize a separate PF, also reduces the effectiveness of passive regeneration and increases the backpressure on the engine. The coating of the SCR catalyst on the PF substrate creates the additional performance reduction in NOx effectiveness as ash is accumulated. Since the ash accumulates near the end of the substrate, the flow of exhaust gases is reduced and then completely blocked. The reduction in flow area reduces the effective catalyst surface area/sites available resulting in reduced NOx effectiveness and passive regeneration while increasing the backpressure. Ash removal on a continuous or increase frequency is desired so as to keep this area active and functioning.
Additionally, the ability to combine multiple aftertreatment modules into a single unit, without sacrificing NOx effectiveness or higher backpressure, requires very high porosity PF substrates. As the porosity is increased, the durability of the substrate is reduced by the high thermal regeneration temperatures and the thermal gradients imposed during the quick ramping up and down of the temperature between normal operating exhaust temperature and the required regeneration temperature set point. The removal of high temperature regeneration allows significant increases in particulate filter substrate porosity. This increase in porosity gives the required volume for additional catalyst and improved NOx effectiveness without an increase in backpressure.
The use of a separate flow through SCR substrate downstream of the SCR catalyst coated PF has been shown, by catalyst company BASF, to increase the NOx performance by over 5% with a 50% increase in the substrate volume. Increased porosity, higher catalyst loading, and oversized substrates provide the similar benefit of increased effectiveness compared to the state-of-the-art would be desired. Particulate filters with a length to diameter ratio greater than 1.3 have been known to be damaged by excessive temperatures at the end of the filter.
In addition to the SCR solution, it has been shown in the prior art that the use of a Passive NOx Adsorber (PNA) can trap NOx as it exits the engine under low temperature operation and then release the NOx as the temperature climbs in operating temperature. Current PNA literature states that desorption of the NOx occurs before current copper zeolite or iron zeolite based metal catalyst are capable of significant NOx reduction effectiveness. For current designs to properly function with a PNA, the PNA needs to begin desorption at 175 degrees Celsius or higher along with the close coupling of the SCR to the PNA, or alternately a SCR capable of significant NOx reduction down to 150 degrees Celsius.
Stationary engines have long utilized a formula of vanadium pentoxide that functions well at temperature ranges between 100 degrees and 350 degrees Celsius. The lower temperature range functions very well for stationary engines where the exhaust temperature is held within a limited range of temperature. The high end of the temperature range does not correspond to the mobile market where exhaust temperatures, near the engine, can reach in excess of 450 degrees Celsius. Since maximum temperature, within the aftertreatment system, is closely linked to the location of the device to the engine, as the distance from the engine increases, radiant and convective heat losses reduce the peak temperature obtained. The aftertreatment industry has focused on the reduction of the distance between the aftertreatment and the engine in order to achieve higher temperatures in the quickest time possible. The placement of the aftertreatment near the engine has been termed close-coupled by the industry. To utilize low temperature SCR catalysts, such as the stationary vanadium pentoxide formula, the opposite must be attained. The further that the device can be positioned away from the engine, the lower the peak temperature. The exhaust temperature near the end of the exhaust system has shown to be a maximum of 203 degrees Celsius without a thermal regenerated PF compared to 418 degrees Celsius during an active thermal regeneration event. In addition to the maximum effective operating temperature constraint of around 350 degrees Celsius for the low temperature SCR catalyst, the ability to inject urea into the exhaust is limited to a temperature high enough for hydrolysis and proper mixing to be completed. The utilization of a PNA near the engine or within the SCR well downstream of the engine allows for the storage of NOx while the engine exhaust, after the PNA, reaches the threshold where the urea can be injected. Further, the increased length between the urea injection site and the SCR substrate improves the NOx reduction effectiveness by allowing for complete hydrolysis and mixing with the exhaust gases before becoming in contact with the SCR catalyst.
Current aftertreatment systems, depending on the countries emissions regulation, further require a particulate filter, to be installed, in order to meet stringent particulate matter mass and number emissions. Since these systems currently utilize a thermal high temperature regeneration to oxidize the particulate matter, temperatures above 500 degrees Celsius are typically generated that would deactivate and damage the low temperature stationary and mobile vanadium V2O5 catalyst formulas. Assisted passive regeneration increases the temperature to a level where passive regeneration can be completed, but this approach is only utilized with systems that utilize significant passive regeneration. In order to keep the temperatures below the SCR temperature constraint, the active thermal regeneration would be required to operate downstream of the low temperature SCR catalyst. Increasing the temperature from a level below the SCR threshold to above the temperature required for carbon oxidation with oxygen would require significant additional fuel. The fuel would be oxidized utilizing a downstream oxy-cat or a fuel burner to provide the high temperatures further decreasing the overall thermal efficiency of the engine. Another potential method would be to place the PF near the engine and actively cool the exhaust or bypass the exhaust around the filter. Cooling the exhaust from these thermal regeneration temperatures to a point where effective SCR NOx reduction can occur would require a cooling system that would require too much packaging space and excessive cost. Another approach would be to bypass the low temperature catalyst at high temperatures. In order to bypass the filter during thermal regeneration without allowing NOx to be released to the environment, the filter would have to additionally be coated with a high temperature SCR catalyst and a separate urea injector installed. The increased complexity of the second sulfur intolerant SCR catalyst and urea injection site would also increase the cost and complexity of the system beyond what the market will bear. Thus, the industry has no answer in order to attain an overlapping operation of the PNA desorption and high SCR effectiveness while providing effective and efficient particulate matter regeneration. Passive regeneration is only possible in applications where the particulate matter generated is lower than the engine's passive regeneration level. Since many applications operate at low temperatures below that required for passive regeneration, an active system is still required for the remaining applications.
In the Illinois Valley Holding Company application PCT_US_053456 (WO 2014025647 A3), a throttle valve is utilized to create a pressure below that of atmospheric conditions within a particulate settling volume. While this approach alters how the differential pressure is attained, the system does not provide SCR NOx reduction capabilities but is a strategy for non-thermal regeneration of the PF.
A global solution, that has high sulfur tolerance with no increase in system temperature during regeneration, is needed so that the development costs can be shared between all markets. Additionally, as vehicles and engines are typically shipped/transferred, during the engine/vehicles useable life, from developed countries to developing countries, the aftertreatment system should be capable of operating on high sulfur fuel. Improved resale value along with the environmental advantages of the engine retaining the original ultra-low emissions will have significant global emissions reduction significance. This global issue is becoming ever more important as the engine mapping for high efficiency and subsequent high engine out NOx output is requiring high aftertreatment emissions reduction effectiveness and subsequent high emitting emissions levels if the system becomes less effective. If the aftertreatment system is not removed and modifications to the Electronic Control Module (ECM) are completed, then the aftertreatment will be poisoned by the higher sulfur level in the developing country's fuel. The NOx emissions, in particular, will be significantly higher than internal combustion engines produced even a decade ago.
Engine aftertreatment has allowed ultra-low emissions even with high engine out emissions. The addition of aftertreatment has reduced the need for sophisticated engine combustion systems. The advantages of high pressure fuel injection for the reduction in particulate matter are reduced if the particulate filter regeneration system is efficient. The SCR catalyst NOx reduction effectiveness allows the advancement of fuel injection timing to further assist in the mixing of low pressure injected fuel and air allowing improved combustion efficiency, complete combustion, and thermally efficient engine performance. The ability to operate with increased engine out particulate levels so as to utilize lower pressure and cost fuel injection systems while still achieving NOx and PM emissions is desired. Engine performance and drivability are enhanced by increasing particulate matter generated by the engine during accelerations. Rich air/fuel ratios allow for increased power output, reducing the need for variable geometry turbochargers and their associated expense. Achieving low emissions with a low cost engine and aftertreatment system lowers the financial barrier to entry into developing markets.
The present invention is directed toward overcoming one or more of these deficiencies of the prior art.